Supercapacitors represent one of the latest innovations in the field of electrical energy storage, and find their place in many applications involving mass energy storage and power distribution. In comparison with classical capacitors, these new components allow a much higher energy density, together with a higher power density. Supercapacitors may be produced based on a double-layer capacitor technology to increase their charge density. However, double layer capacitors have a relatively low maximum voltage. This necessitates a series connection of supercapacitive elements or cells to support operation at higher voltages in order to reach an acceptable power conversion efficiency.
As voltages on individual double-layer capacitors depend on charges and may differ from each other, a supercapacitor system composed of series-connected capacitor cells needs a circuit for equalizing voltages on individual capacitor cells. Conventionally, voltage equalizing circuits include passive resistors or zener diodes connected in parallel to the capacitor cells. However, such circuits suffer from high power loss.
Other conventional voltage equalizing circuits use active balancing of voltages on individual capacitor cells. Active voltage balancing may be achieved using a buck/boost converter, a fly-back converter or an operational amplifier circuit. However, rather complex circuitry is required to actively balance voltages on capacitor cells.
Therefore, it would be desirable to create a simple and efficient circuit for charging series-connected capacitors so as to provide automatic voltage balancing.